The Third Generation (3G) communication system consists of a Core Network (CN), a Radio Access Network (RAN) and User Equipment (UE). The main function of the CN is traffic forwarding and addressing between the UEs, and the CNs are divided into a Circuit Switch (CS) domain and a Packet Switch (PS) domain. The CS domain features stable transmission characteristics, low time delay and bandwidth assurance owing to being based on circuit switch. The CS domain can provide 64K bandwidth resource to each UE.
The 3G network can provide mobile Video Phone (VP) call service. The mobile VP call service is a new mobile service which may deliver video contents to users via the 3G network and 3G equipment, so that users may not only hear from each other but also watch the video image of the counterpart in a real time manner. In the 3G network in R99 and R4 releases, like the traditional voice service, the mobile VP call service adopts narrow-band circuit bearer. The narrow-band circuit bearer indicates that traffic is transmitted on a narrow-band circuit. In order to implement multimedia communication on the narrow-band circuit, i.e. to implement real time video and voice transmission, the ITU-T defines H.324 series of low-speed multimedia interworking protocol family dedicated to the mobile VP call service. The protocol family includes H.245 protocol (Control Protocol for Multimedia Communication), H.223 protocol, etc. The H.245 protocol specifies signaling adopted for implementing multimedia communication. The H.223 protocol specifies a multiplexing method for implementing multimedia communication. Audio data and video data, etc., in the multimedia communication is combined into H.223 multiplexing stream with a specific multiplexing method. According to the H.324 protocol, the 3G equipment may be deemed as an H.324 entity, which adopts narrow-band circuit bearer, i.e., the mobile VP call service is developed in the CS domain of the 3G network. The H.324 entity multiplexes audio stream, video stream and control stream, which are in compliance with the H.245 protocol and are involved in the mobile VP call service, onto a narrow-band circuit with a 64 kbit/s bandwidth via the H.223 protocol. The mobile VP call service is abbreviated as VP call service hereinafter.
In the 3G network, a flow of the VP call service may include following two phases.
The first phase: The narrow-band circuit for bearer is activated via narrow-band Integrated Services Digital Network User Part (ISUP) signaling.
The second phase: Parameters are negotiated between two parties, H.324 entities, by means of the activated narrow-band circuit. Content to be negotiated includes: H.245 protocol-compliant audio and video codec modes used in a call, H.223 protocol-compliant multiplexing parameters used for audio and video coding and decoding, and multiplexed audio and video logical channels, etc. The purpose of negotiation is to determine communication parameters required in the call of the two parties, H.324 entities. On a successful negotiation, the two parties, H.324 entities send audio and video stream to each other according to negotiated parameters, to implement the VP call service.
In the traditional 2G network, on the premise that a call is not disconnected, the switch of a call object of a user is implemented by a circuit switch device changing a narrow-band circuit for bearer. For example, in the customized ring back tone service in the traditional 2G network, a calling user A1 originates a call to a called user C1. The call arrives at a Mobile Switching Center (MSC), in which it is queried that the called user C1 has registered the customized ring back tone service, and the MSC routes the call to a customized ring back tone center. The customized ring back tone center originates a call to the called user C1. After the called user C1 rings, the customized ring back tone center plays audio customized ring back tone subscribed to by the called user C1 to the calling user A1. In this case, the call object of A1 is a device B1 responsible for playing the audio customized ring back tone in the customized ring back tone center. After the called user C1 performs an off-hook, a circuit switch device in the customized ring back tone center changes the narrow-band circuit connection directly on the premise that the call is not disconnected, so that the call object of the calling user A1 is changed from the device B1 to the called user C1, thereby the calling user A1 is connected with the called user C1.
The above processing methods may cause problems in the H.324-compliant VP call service. For example, in the CS domain of the 3G network, H.324 entities, such as A2 and B2, implement the VP call service after the parameter negotiation. If it is needed that the call object B2 of A2 is changed to be another H.324 entity, C2, on the premise that the call is not disconnected, the changing of the involved narrow-band circuit is the same as that in the 2G network for the first phase of the VP call service, i.e. the circuit switch device changes the narrow-band circuit connection directly. However, a problem may arise to the parameter negotiation between H.324 entities involved in the second phase. For the A2 in a normal call state after a negotiation, if the call object B2 in a normal state is changed to C2 in an initial state, the A2 does not know the changing of the call object and deems that the B2 goes abnormal, which affects the normal implementation of the VP call service between the A2 and the C2, thereby causing the call to be disconnected.
In the prior art, a Video Interworking Gateway (VIG) is utilized to implement the VP call service between an H.324 entity and a broadband resource device. The VIG is a physical entity capable of providing video interworking gateway resources. The video interworking gateway resource is a logic entity embodying the function of the Video Interworking Gateway. One VIG may provide multiple video interworking gateway resources. It is certain that the VP call service through the VIG involves one end of the H.324 entity, which generally utilizes a narrow-band circuit bearer based on the time division multiplexing (TDM) technology, and the other end of a broadband resource device, which generally utilizes a broadband IP bearer based on the Real-time Transfer Protocol (RTP). The broadband IP bearer means traffic is transmitted via a broadband IP network. If the VIG is utilized to implement the VP call service between the H.324 entity and the broadband resource device, the process is as follows: Parameters are negotiated between the VIG and the H.324 entity, the H.324 entity is connected to the broadband resource device on a successful negotiation, the VIG utilizes a video interworking gateway resource to perform burdensome media stream coding and decoding between two heterogeneous networks during the implementation of the VP call service between the H.324 entity and the broadband resource device. However, as shown in FIG. 1, the VP call service supported by the VIG involves one end of the H.324 entity in the CS domain of the 3G network and the other end of the broadband resource device, and the VIG does not need to provide any service for the VP call service between two H.324 entities.